Modification of Biochar Formation during Slow Pyrolysis in the Presence of Alkali Metal Carbonate Additives

In this work, slow pyrolysis of sawdust of Eucalyptus pilularis biomass and ternary molten carbonate eutectic [Li2CO3, 43.5%; Na2CO3, 31.5%; and K2CO3, 25% (mole percentage)] in thermogravimetric analysis at three different temperatures, 600, 750, and 900 °C, was studied. These salts affect the slow pyrolysis process, including changes in the volatile release mechanism and the morphology of remnant char material. The initial results show that, in the presence of molten carbonate, biomass particles make bubble-shaped larger particles, which result in less volatile emissions and more char residue. It is suggested that the ternary eutectic has a chemical diluent and catalytic role, particularly in the case of higher salt doping. Results from scanning electron microscopy images give strong evidence that molten carbonates capture volatiles inside swelling carbon particles, which causes the generation of various sizes of pores as well as char-making reactions, and at a higher temperature, the bubble-shaped particles will rupture. Swelling of this nature has previously only been observed clearly in coal precursors; however, this is the first observation in a biomass-based system. Also, at a temperature above 750 °C, decomposition of molten carbonate generates CO2 and carbon/carbonate gasification produces CO as well as a more "activated" biochar.